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Kevin Bush1 Kyle Frohna1 Hsin-Ping Wang1 Rebecca Belisle1 Zhengshan Yu2 Zachary Holman2 Michael McGehee1

1, Stanford University, Stanford, California, United States
2, Arizona State University, Tempe, Arizona, United States

The wide, tunable bandgap of metal halide perovskites holds the promise of boosting the efficiency of silicon by employing them as the wide gap absorber in tandem solar cells on silicon. This offers a pathway to surpassing fundamental efficiency limits on single-junction solar cells by extracting a portion of photogenerated carriers at a higher voltage and thus enabling the realization of the next generation of low cost tandem photovoltaic cells. Perovskite silicon tandems have recently achieved record efficiencies of 23.6% for monolithically integrated1 and 26.4% for mechanically stacked configurations2. Additionally, recent developments in the stability and efficiency of low bandgap tin-based perovskites have drawn great attention for making highly efficient and potentially low cost perovskite-perovskite tandems3.
However, while the bandgap of perovskites can be easily and continuously tuned between 1.5 and 2.3eV by the substitution of bromide for iodide, open circuit voltages have not increased linearly with bandgap, largely negating the benefit of bandgap tuning. One major cause for this saturation in open circuit voltage is photoinduced halide segregation, referred to as the Hoke effect4, where halides segregate into lower bandgap I-rich and higher bandgap Br-rich regions upon illumination. Photogenerated carrier are funneled towards these lower bandgap regions, which act as recombination centers, resulting is a loss in open circuit voltage.
In this work, we explore two strategies to mitigate halide segregation in wide bandgap perovskites and target bandgaps of 1.68eV and 1.75eV due to their relevance in tandems. First, we characterize the FA/Cs and I/Br compositional space and find that higher open circuit voltages are achieved and photostability is improved when the band gap is attained by raising the Cs fraction rather than relying on the Br fraction. Second, we find that halide segregation is largely mediated by surface trap states and the passivation of surfaces can reduce the rate of halide segregation. Combining these two insights, we fabricate monolithically integrated perovskite/silicon tandems with high and stable open circuit voltages, indicating sufficient suppression of halide segregation. Our optimized 1.68eV bandgap perovskite enables a matched current density of 19mA/cm2 to realize >25% efficient monolithic perovskite tandems.

1. Bush, K. A. et al. 23.6%-Efficient Monolithic Perovskite/Silicon Tandem Solar Cells with Improved Stability. Nat. Energy 2, 17009 (2017).
2. Duong, T. et al. Rubidium Multication Perovskite with Optimized Bandgap for Perovskite Silicon Tandem with over 26% Efficiency. Adv. Energy Mater. 1700228, 1–11 (2017).
3. Eperon, G. E. et al. Perovskite-perovskite tandem photovoltaics with ideal bandgaps. Science. 9717, 1–9 (2016).
4. Hoke, E. T. et al. Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chem. Sci. 6, 613–617 (2015).

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